Abstract
O NE of the main mechanisms responsible for the generation of turbofan noise in flight is the interaction of the unsteady rotor wake field with the fan stators. Analysis of the strength of this interaction noise source requires a knowledge of the rotor-produced flow disturbances as a function of radius and downstream distance. Thus the validity of noise-related design choices such as rotor-stator spacing and the determination of the position of the blade span most responsible for noise generation depend on the rotor wake flow field characterization/The parameters usually described are the magnitude of the wake defect and its width. The purpose of the present experiment is to determine these parameters along with the harmonic content of the rotor wakes in order to provide wake parameters of interest to acousticians and fan designers. Contents Existing rotor wake data were primarily obtained from lowspeed laboratory fans.1'2 The present study was conducted to measure wake properties under conditions that are more representative of practical turbofan operation. The experiments were performed on a 0.5-m-diameter fan with a 1.2 pressure ratio in the NASA Lewis 9x15 ft anechoic wind tunnel.3 The fan was tested at a tip speed of 197 m/s, equal to 96% of the design speed (8020 rpm.) Rotor wake properties under static (£7=0) and forward velocity conditions (£7=41 m/s) were measured with a stationary cross-film anemometer circumferentially located midway between two stator vanes and oriented to measure the streamwise and upwash components of the velocity downstream of the rotor, as indicated in Fig. 1. The anemometer signals were analyzed in a Fast Fourier analyzer, utilizing an ensemble averaging technique in which the analyzer was triggered by a once-per-revolution signal. Data from radial locations at the midspan and near the fan tip are presented for three downstream locations ranging from 0.54 to 1.77 rotor
Published Version
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